1. Field of the Invention
The field of the present invention relates to overhead valve engines, such as overhead cam engines, for use in a variety of applications such as lawnmowers, other lawn and garden implements, or in small utility vehicles such as riding lawnmowers, lawn tractors, and the like. In particular, the invention relates to a drive train for such engines where the intake and exhaust valves are actuated by rocker arms which are rotated by a cam ring, the cam ring driven by a timing shaft with gears at each end to connect the cam ring to the crankshaft.
2. Description of the Related Art
Prior known engines that contain drive trains of an overhead valve design are well known in the art. In one arrangement, the crankshaft supplies rotary mechanical motion to a camshaft by means of a belt, chain, or the like through a pulley or sprocket mounted on the camshaft. The camshaft includes one or more lobes that actuate the intake and exhaust valves in the cylinder head either directly, or indirectly through the use of rocker arms, push rods, or other similar means.
In another arrangement, the crankshaft drives a camshaft located near the crankcase through a gear set. The camshaft includes one or more lobes which actuate a pair of cam followers mounted for rotation on a cam follower shaft. The cam followers in turn reciprocate a pair of push rods extending therefrom to the cylinder head, which in turn rotate a pair of rocker arms mounted in the cylinder head to open and close intake and exhaust valves.
A disadvantage with the first arrangement is that the location of the camshaft in the cylinder head increases the width of the cylinder head due to the lateral space between cam lobes and between a cam lobe and the pulley or sprocket which is driven from the crankshaft. In addition, the location of the camshaft directly above the valves, and the relatively large size of the pulley or sprocket necessary for speed reduction from the crankshaft increase the length of the cylinder head. Further, the belt or chain which drives the camshaft from the crankshaft is prone to wearing or breakage.
A disadvantage with the second arrangement is that the several components of the drive train, including the camshaft, camshaft lobes, cam followers, cam follower shafts, push rods, and rocker arms tend to increase the size, complexity, and cost of the engine, as well as the difficulty of assembly and likelihood of failure of one of the components. The relatively large number of reciprocating parts additionally increases engine noise.
An additional disadvantage of each of the above arrangements is that they permit little variation in the location, size, and arrangement of the components thereof, and therefore restrict design freedom.
What is needed is an engine having a drive train which is compact, to allow a smaller cylinder head, shorter engine length, and an overall reduced engine silhouette.
Another need is for an engine having a drive train including a minimum number of durable components and a minimum number of reciprocating components.
A further need is for an engine in which the location and size of the drive train components may be varied to allow a large degree of design freedom.